Background: The Repeat Expansion Diseases are caused by the intergenerational expansion of a specific tandem repeat. Expansion of a CGG?CCG-repeat in the 5' UTR of the FMR1 gene is associated with 3 different clinical presentations: Individuals with 60-200 repeats, the so-called premutation allele, are at risk for Fragile X-associated tremor-ataxia syndrome whose symptoms include dementia as well as bowel and urinary incontinence. Female carriers of premutation alleles are also at risk of premature ovarian failure. Furthermore, in females, the premutation allele can undergo expansion on intergenerational transfer that can result in their children having alleles with >200 repeats. This expanded allele is known as a full mutation and individuals who inherit such alleles almost always have Fragile X mental retardation syndrome (FXS). FXS is the most common cause of mental retardation and the most common known cause of autism. Hyperphagia and obesity are common comorbid features. FXS symptoms result from some combination of repeat-induced gene silencing and difficulties in translating any residual FMR1 mRNA. This results in a deficiency of the protein product of this gene, FMRP, a protein involved in the regulation of translation of certain mRNAs. GAA?TTC-repeat expansion in the first intron of the frataxin gene causes a deficit in mRNA for Frataxin a protein involved in iron homeostasis in the mitochondria. This deficit produces Friedreich ataxia (FRDA), a progressive degenerative disease associated with cerebellar dysfunction, hypertrophic cardiomyopathy, and diabetes. We are interested in both the mechanism of expansion and the consequences of expansion in these disorders.[unreadable] [unreadable] Progress report: We had previously generated FXS premutation mice containing 120 CGG?CCG-repeats in the 5? UTR of the endogenous murine Fmr1 gene. Like humans with the same number of repeats, these mice produce elevated levels of Fmr1 mRNA that recent data, from my group and elsewhere, suggests is toxic. Microarray analysis shows a number of interesting changes in gene expression in the brains and ovaries that are consistent with some of the pathological changes that we can see in these mice. These pathological changes include the accumulation of ubiquitin and lamin A/C positive intranuclear neuronal inclusions in brain that are reminiscent of those seen in human carriers of premutation alleles. We have also identified a CGG-RNA binding protein in brain that may be associated with the RNA toxicity. We have also shown that mice with premutation alleles have a discernable decrease in the Fmr1 gene product, FMRP, in particular parts of the brain. This finding has consequences for some of the symptoms seen in premutation carriers. We have also shown that the GAA?TTC-repeat expansion responsible for FRDA has epigenetic effects resulting in the accumulation of histone modifications characteristic of transcriptionally silenced genes. Similar chromatin modifications are found at the FMR1 promoter in FXS. This raises the possibility that disease pathology in FXS and FRDA have a similar underlying mechanism. We are currently in the process of studying the molecular triggers for this gene silencing in cell lines from patients with FXS and FRDA as well as in a mice model we have generated for FXS. We have also identified a number of small molecules capable of affecting gene expression in FXS. This is important in that it throws light on the mechanism of gene silencing and also suggests potential approaches to reversing or ameliorating disease symptoms.